1. Field of the Invention
This invention relates to an electromagnetic flowmeter for measuring the flow rate of a fluid such as a conductive fluid, and more particularly to an electromagnetic flowmeter having a noise elimination function of effectively eliminating a spike noise component contained in the flow rate signal, particularly, spike noises occurring at periods close to the excitation period and maintaining the response to the flow rate.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing an example of the construction of a conventional electromagnetic flowmeter.
The electromagnetic flowmeter includes a detector 1 and a converter 11.
The detector 1 includes a measurement tube 2, electrodes 4 mounted on the inner wall of the measurement tube 2, and an exciting coil 3 for applying magnetic flux to the measurement tube 2.
The converter 11 includes an exciting circuit 5 for supplying a current to the exciting coil 3 and periodically switching the direction of the current, an amplifier 6 for amplifying electromotive force (flow rate signal) from the electrodes 4, an A/D converter 7 for sampling an amplified flow rate signal for a preset period of time and converting the same into a digital value, a control circuit 8 for controlling the switching operation of the excitation direction and the sampling timings, an arithmetic circuit 9 for processing the flow rate signal converted in the digital form, and an output circuit 10 for outputting the flow rate signal.
With the above construction, the reason for periodically switching the direction of a current flowing in the exciting coil 3 to apply alternating magnetic flux is that an influence on the flow rate signal given by a DC-like polarization voltage due to the electrochemical action occurring in the electrodes 4 of the detector 1 can be reduced (that is, an influence by the DC noise can be prevented by deriving a difference between flow rate signals sampled in both of the exciting directions).
In the above electromagnetic flowmeter, an indication (output) may become sometimes unstable by a spike noise caused when solid materials in the fluid collide against the electrodes 4.
The fluctuation of indication can be suppressed to some extent by increasing the damping constant and averaging the flow rate signals, but with this method, the response of the electromagnetic flowmeter must be sacrificed, and therefore, its inherent performance cannot be fully exhibited.
In the well known measurement technique, the technique for effecting the arithmetic operation by excluding measurements which exceed a preset range with respect to the input range of a measuring instrument which effects measurements at preset periods or replacing the same by simulated values is considered, but if spike noises occur at periods close to the excitation period, the measurement for each excitation period always receives the influence of the noise and is not effective.
As described above, in the conventional electro-magnetic flowmeter, the response to the flow rate is degraded or it is influenced by the spike noise and the indication (output) thereof becomes unstable.